Analog and digital electronic circuitry and attendant wiring may encounter serious operating difficulty in the presence of radiated electromagnetic fields. When such fields interfere with the operation of electronic circuitry, they are generally referred to as Electromagnetic Interference (EMI) fields. The circuits and attendant wiring may be shielded and filtered to provide some immunity to large electromagnetic fields, however, it is not possible or practical to design the circuitry and attendant wiring so as to ensure immunity to such fields. Indeed, shielded cables can sometimes be more susceptible to EMI problems than unshielded wires, because bending of a shielding cable, or other abuse, may produce a pin hole in the shielding, which can cause the shielding to act as a waveguide for EMI, with consequent deleterious effects.
Methods and apparatus, therefore, are required to test the susceptibility of the devices, such as integrated circuits, electrical components, and the like, and systems, such as automotive electrical systems, for both commercial and military applications, to electromagnetic fields. The term "system" is employed hereinafter to broadly describe any device or system, such as, but not limited to those described above, that may be tested for susceptibility to radiated electromagnetic fields.
Electromagnetic field testing is typically performed in shielded enclosures, or "screen rooms," which provide an environment wherein ambient electromagnetic fields are eliminated and a controlled field is produced, in order to determine, with certainty, the effect of a given level (measured in volts per meter) and frequency of electromagnetic fields on the system undergoing test. Apparatus typically used inside the shielded enclosure includes current probes attached to a system harness wire and a transmitter which sends the signals detected by the probes to a receiver outside the shielded enclosure, where the effects of the electromagnetic fields on the system are determined.
To ensure the integrity of the shielded enclosure and the results of the tests, any voltage measuring apparatus within the screen room should minimally perturb the controlled electromagnetic fields and should not inject EMI into the system. For example, any test apparatus which might reradiate electromagnetic fields impinging on the device under test, or might itself be susceptible to such fields or otherwise inject any noise into the system, must be avoided.
In U.S. Pat. No. 4,939,446, by Wesley A. Rogers, the inventor of the present invention, the performance of a system under test is observed in a shielded enclosure, with and without the presence of controlled E fields radiated by one or more antennas. The problem of re-radiation of fields, or the injection of noise into the system by the test equipment itself, is eliminated through the use of non-metallic overdamped conductors, and a hybrid electrical/optical transmitter and optical cable used to transmit voltage signals from the system to a receiver monitored by an oscilloscope located outside the shielded enclosure. The approach set forth in the above patent allows accurate testing of systems in a controlled electromagnetic environment, since the overdamped conductors are transparent to the electromagnetic fields. The entire disclosure of U.S. Pat. No. 4,939,446 is incorporated herein by reference thereto.
The standard operating procedure in determining the susceptibility of systems to radiated electromagnetic fields is to place the system in a shielded enclosure, as mentioned above, and to test the susceptibility of the system over a wide range of radio frequencies, for example from 10 KHz to 18 GHz or more. The radiated field is swept through the desired frequency range, at a range of levels, e.g. between 1 v/m and 300 v/m, and the susceptibility of the system is determined over the range of frequencies. If, for example, it is determined that the system is susceptible to a radiated field at 2 GHz and a certain volt per meter level, a suitable filter or other expedient can be placed in the system, and the system retested at that frequency and level to see whether the filter is effective in removing the system's susceptibility. If the system is still susceptible, the filter must be changed, or another approach must be adopted.
This trial and error technique must be used for each frequency and level at which there is a susceptibility problem. Moreover, the trial and error testing and retesting has to take place in a shielded enclosure, as referred to above, since the FCC prohibits the generation of the antenna-radiated fields necessary to conduct open air testing, and since there is no way to tell what effect, if any, ambient fields might have on the system--the controlled environment provided by the shielded enclosure is necessary to accurately determine susceptibility problems. The testing and retesting of systems, in order to troubleshoot and alleviate susceptibility to radiated fields, usually requires between one and two weeks for each system. The rental of a shielded enclosure, such as a screen room, can cost between $1,000-$3,000 a day, and thus runs into considerable expense, and constitutes a bottleneck for the development of new systems. Furthermore, even with this procedure, there is no indication of the voltage level of EMI induced in the system that caused a failure. A test technique that allows an accurate determination of that level has long been desired.